Circuit boards are stiff structures comprised of a dielectric substrate upon which is provided an electronic circuit. Many different techniques have been employed for providing the electronic circuit on the dielectric substrate. This invention is not concerned with the formation of this circuit. Printed circuit techniques well known in the art may be used in practicing this invention. This invention is concerned with the dielectric substrate and includes the relationship of the substrate to the circuit.
Many circuit boards employ fiber reinforced thermosetting resins and thermoplastic polymers as the dielectric substrate upon which the circuit is provided. Such structures have a relatively high weight to volume ratio because of the weight introduced by the fibers, typically glass fibers, and the resin or plastic component. Certain engineering polymers do not require the use of fiber reinforcement for some circuit board applications, but they still introduce a significant weight to volume ratio to the circuit board.
Circuit boards have become significant components in terms of the weight of many articles utilizing computers. In a microcomputer, they may constitute, exclusive of the housing, the single heaviest component(s) in the computer. Computers are now significant components of machines that are transported or otherwise caused to be moved. As a result, the weight introduction of the circuit board to the item utilizing it has become a serious design and materials problem. As more and more computer functions are added to equipment, the weight of the circuit board(s) becomes a materials and design issue not easily resolved. As printed circuits become larger and more complexed, and then multiplied in use, it is evident that reducing the weight of circuit boards is most desirable thing to do.
Printed circuit boards have many important property requirements. They must be stable structures under a wide variety of service requirements. For example, they must be stiff structures that do not warp or buckle under production and use conditions. They must be plateable under relatively severe plating conditions used to create the circuit, without warpage or delamination. They must accept the ambient conditions involved in commercial uses without introducing flaws that affect the circuit's performance.
It is now apparent that there is a commercial need for circuit boards with a combination of such features as
lower weight PA1 stiff structures PA1 high stiffness to weight ratio PA1 high strength to weight ratio PA1 high lamination strength PA1 environmental stability PA1 platability without warpage or buckling PA1 low dielectric constant PA1 faster signal speed PA1 capable of achieving improved thermal conductivity in select situations PA1 enhanced fire retardency PA1 (1) Adding one ply of 0.020 inch SynCore.RTM. and eliminating one ply of prepreg does not change the weight or cost significantly, but nearly doubles the flexural rigidity. PA1 (2) Adding one ply of 0.020 inch SynCore.RTM. and eliminating three plies of prepreg sharply decreases the cost and weight with a small decrease in rigidity.
SynCore.RTM., sold by Dexter Adhesive and Structural Materials Division, Dexter Corporation, Pittsburg, Calif. 94565 U.S.A., is a syntactic film that takes the place of prepreg plies in stiffness critical structures. This syntactic film is a composite material consisting of microballoons in a matrix resin. A wide variety of microballoons and matrices can be combined to make SynCore.RTM. materials. Glass is the most common microballoo material of construction, but quartz, phenolic, carbon, thermoplastic and metal coated microballoons have been used. Epoxies curing at 350.degree. F. (177.degree. C.) and 250.degree. F. (121.degree. C.) are the most common matrix resins, but matrices of bismaleimide (BMI), phenolic, polyester, PMR-15 polyimide, cyanate esters, and acetylene terminated resins have been used to produce SynCore.RTM. products. In addition, these types of syntactic films may be formed using thermoplastic polymers to form the composite structure. As a result of the wide variety of materials that successfully make SynCore.RTM. products, they are tailorable to a variety of applications. There is a version of SynCore.RTM. available that will cocure with all known available heat-cured composite laminating resins. In that respect, syntactic films made from thermoplastic polymers compatible with other thermoplastics provided within a more complex composite structure is readily achievable.
SynCore.RTM. provides a unique thin film form in isotropic film structures. SynCore.RTM. allows sandwich core concepts to be used in a thinner dimension than previously possible. The thickness limit on honeycomb cores is approximately 0.125 inch. SynCore.RTM. is available in 0.007 to 0.125 inch thicknesses but can be made in thinner or thicker sheet forms. Other core materials such as wood and sheet film can be made thin, but are not drapable and generally require an expensive/heavy adhesive film to bond to the partner composite components. In addition, SynCore.RTM. possess excellent uniformity in thickness which provides the ability to assure quality for the composite in which it is used as a component. SynCore.RTM. is typically used to replace prepreg plies where the intent is to increase stiffness by increasing thickness.
Designing with Syn Core.RTM. is straightforward because all of the analysis methods that apply to other core materials such as honeycomb apply to it. Flexural stiffness of flat plates and beams increases as a cubic function of thickness allowing a lighter, stiffer lamination than could be made from prepreg plies alone. Since SynCore.RTM. on a per volume basis typically costs less than half of a comparable carbon prepreg, it also leads to a lower cost lamination. This is illustrated by the following:
(3) Adding one ply of 0.040 SynCore.RTM. and eliminating three plies of prepreg provides lower weight, cost and sharply increases rigidity.
(4) The introduction of unidirectional tape allows a further increase in performance at lower cost and weight at nearly the same thickness.
(5) A hybrid tape/fabric/SynCore.RTM. construction gives a very attractive set of weight and cost savings coupled with a 3.4 times increase in flexural rigidity.
SynCore.RTM. has been recommended for thin composite structures in any application where flexural stiffness, buckling, or minimum gauge construction is used. It has been shown to save weight and material cost in carbon fiber composites. It has been been offered to save weight at approximately the same cost in the case of glass fiber composites.
The manufacturing methods for employing SynCore.RTM. are very similar to those used for prepregs. Because it is not cured, it is tacky and very drapable when warmed to room temperature and is easier to layup than a comparable prepreg ply. It can be supplied in supported forms with a light weight scrim to prevent handling damage when it is frozen. It requires cold storage like prepregs, usually 0.degree. F. (-17.7.degree. C.) or below. The various SynCore.RTM. typically have a room temperature out-time that is much longer than their companion prepregs. Because the microballons provide a large degree of flow control, SynCore.RTM. does not show any unusual migration during cure when normal laminate layup and bagging procedures are used. SynCore.RTM. is less sensitive to cure cycle variations than prepreg making the controlling factor the composite cure cycle selection. It will cure void free under full vacuum or low (e.g. about 10 p.s.i.) autoclave pressure. It has been cured at up to about 200 p.s.i. without exhibiting balloon crushing.
In a typical application, a sandwich of SynCore.RTM. and prepreg, such as a thicker layer of SynCore.RTM. between two thinner layers of prepreg, are held together under heat and pressure to cure the structure into a strong panel. Typical sandwich constructions of this nature are shown in U.S. Pat. Nos. 4,013,810, 4,433,068 and 3,996,654. Such composite structures typically are produced in flat sheets and in separable molds to obtain various desired shapes. Moreover, SynCore.RTM. can be precoated on the laminate prepreg before the prepreg is stacked into the composite form, see U.S. Pat. No. 4,284,679, or stacked separately with the prepreg.
Syntactic films may be made by a variety of methods. Typically, the resin or plastic that is used to form the film is first blended with the microballoons and the combination is calendared out as a uniform thin sheet having the uniform dimensions described above. In some instances, the resin or thermplastic polymer containing the microballoon loading is converted into a hot melt from which it may be coated, typically by roller or knife coating, onto the desired substrate. In this fashion, one may produce a thin uniform layer of the syntactic film onto the substrate to which it has been deposited. Hot melt coating is effected without the use of solvent(s) or minimal amounts of solvent(s). Where hot melt coating of thermosetting resins containing microballoons is employed, it is desirable to control the viscosity of the resin by the use of reactive diluents thereby avoiding the need to use solvents in the resin system. This eliminates enviromental and safety problems in applying the syntactic film to the desired substrate. It is well recognized that the pot life of thermosetting resins in hot melt applications will often dictate the hot melt conditions.